We have recently developed an experimental system in which the nature of the transition between hormone-dependent and autonomous states in cells from an androgen-dependent mouse mammary adenocarcinoma (Shionogi carcinoma 115) can be examined using genetic and biochemical approaches. This system involves the use of cloned, cultured androgen-dependent and autonomous cell lines and culture conditions under which growth of dependent cells is dependent on the presence of added androgen. It is possible to determine the proportion of dependent and autonomous cell in mixed populations and to grow the cell lines as tumours in mice. We propose to use this system to analyze the nature of the transition between dependence and autonomy and to study the mechanism of androgen regulated cell proliferation.